1. Field of the Invention
The present invention relates to a process for plating stainless steel directly with gold, by which a gold plating excellent in adhesion, appearance and corrosion resistance can directly be formed on the stainless steel without corrosion of the metal texture of the stainless steel.
2. Description of the Prior Art
For gold plating of stainless steel, cleaning and activation are indispensable as preliminary treatments as in the case of gold plating of ordinary metals. Furthermore, in the case of gold plating of stainless steel, it is necessary to completely remove a special passive state film present on the surface of the stainless steel.
Even if this passive state film is removed by an acid solution, this film is readily formed again on the surface of the stainless steel in water or air and the adhesion of the formed plating is degraded by this passive state film. Therefore, it is indispensable to prevent re-formation of this passive state film during the steps between the activating treatment and the plating operation.
One of main causes of re-formation of a passive state film is that when a stainless steel is washed with water after the activating treatment, water flows down from the surface of the stainless steel and a phenomenon of so-called "water breaks" takes place, which renders the surface or the stainless steel dry. Accordingly, it also is necessary to prevent occurrence of this undesirable phenomenon.
A passive state film formed on the surface of stainless steel is not composed of a simple metal oxide but is an amorphous film composed of an alloy of chromium and iron, which has properties similar to those of glass. Moreover, this film is very thin and the thickness is ordinarily in the range of from 30 to 50 .ANG..
This film exerts a peculiar anticorrosion effect on stainless steel, and the film impedes the plating operation. Accordingly, even if a stainless steel is subjected to a surface-activating treatment applied to ordinary metals such as copper and iron, it is impossible to form a good plating on the surface of the stainless steel.
Various research experiments have heretofore been made on methods of gold plating of stainless steel, but a good method for direct gold plating of stainless steel has not been developed. The following two methods are now adopted for gold plating of stainless steel despite various defects involved therein.
According to the first method, as pickling solution is formed by mixing an acid solution comprising hydrochloric acid or sulfuric acid alone or a mixture thereof at a high concentration with an other organic or inorganic acid, a stainless steel is dipped in the so formed pickling solution at a high temperature of 70.degree. to 90.degree. C. to effect activation, and then, the activated stainless steel is subjected to electroless copper plating, nickel plating and finally gold plating (triple-plating method) or the activated stainless steel is subjected to electrolytic or electroless nickel plating and finally gold plating (double-plating method).
According to the second method, a stainless steel is subjected to cathode electrolytic activation using a mixed acid comprising 30 to 40% by weight of hydrochloric acid and 1 to 7% by weight of hydrofluoric acid to effect activation and then, the activated stainless steel is directly plated with gold.
These two methods, however, have unavoidable defects in common. Since a strong acid is used for activation, a passive state film present on the surface of a stainless steel can be removed, but also the texture of the stainless steel is corroded by such strong acid. This over-pickling phenomenon is especially conspicuous in the second method since hydrofluoric acid is used, and the mirror-polished surface of the stainless steel is clouded and the surface appearance is degraded.
When a stainless steel having the surface thus roughened is subjected to gold plating, the plated surface becomes cloudy and a beautiful gloss plating cannot be obtained. Furthermore, this surface roughening results in formation of pinholes on the plated surface, and such defects as reduction of the corrosion resistance and acceleration of rusting arise.
When an ultrafine stainless steel wire for an electronic device part or the like is plated with gold, the wire diameter is reduced and made irregular by over-pickling, and a stainless steel wire having a diameter of about 10 .mu.m is liable to be dissolved out by excessive activation.
As is seen from the foregoing description, conspicuous over-pickling takes place in a stainless steel if strong acid dipping or cathode electrolytic activation is used. This is due to the selective corrosion of chromium in a stainless alloy by the acid solution. More specifically, chromium molecules are dissolved out from the steel surface to roughen the surface.
When stainless steel is subjected to under-plating with nickel (the plating layer has a high hardness and is poor in ductility), cracks are readily formed on the nickel layer upon bending, and also the top layer of gold is cracked by cracking of the nickel under-plating layer, resulting in drastic reduction of the electric conductivity and corrosion resistance. When the plated stainless steel is used for an electronic device part, the properties of an electronic device are adversely influenced by the magnetic characteristic of nickel. Therefore, nickel under-plating is not preferred.
I did research with a view to developing an excellent gold plating process capable of forming a gold plating layer on stainless steel without roughening the surface of the stainless steel or degrading of the mirror-polished surface and also without reducting the diameter in the case of an ultrafine stainless steel wire, while eliminating the foregoing defects of the conventional gold-plating techniques, and I have now completed the present invention.
Ideal conditions for direct gold plating of a stainless steel are as follows. First of all, only a very thin passive state film formed on the surface of the stainless steel is removed while preventing intrusion of acids into the texture of the stainless steel and thus inhibiting selective corrosion of chromium. In the second place, even if water washing is carried out after the activating treatment, occurrence of an undesirable phenomenon of water breaks is effectively prevented and a completely activated state is produced on the surface of the stainless steel. In the third place, this completely activated state can be maintained until the stainless steel is subjected to the gold plating operation. If these conditions are satisfied, direct gold plating of stainless steels will ideally be accomplished.